Revit User's Manual/Area Plans
Unlike in drafting progams where areas are cacluated using poly-lines, a pad of paper and a caculator, Revit uses area plans and schedules. An area plan is a view of the model where you can associate area boundaries and choose the areas that these boundaries define. The area information automatically populates your area schedule.
Area Types 
Revit calculates areas based on the area type you choose when creating your area plan. For example, Gross Building area calculates to the exterior of the area walls, wheras Rentable area calculates to the interior. It is important to choose your area type appropriately; if you are trying to find out the footprint of a building, the Rentable area type would be the wrong one to choose.
Scheduling Areas 
As Gross Building area is often used to caculate both floor plate area (square footage) as well as site area, it is useful to use the comment parameter to keep track of these separately. You may want to use a comment such as "Square Footage" or "Above Grade" for your building areas, and then "Site" for your footprint and lot areas. When you set up your schedules, you can filter out the areas that don't have the appropriate comment. For example, your site schedule may have two areas, the site area and the building footprint. Make sure the comment field for both of these areas is "Site", and then set up a filter in the schedule properties to exclude any area that does not have the comment "Site". The schedule will then only report the relevant site areas. You may want to hide the comment field in the appearance tab because you are only using it to sort and doesn't actually need to appear in the schedule on the plans.